Constant reflux

Operating the column at a constant reflux rate results in a distillate with a continu- 

The sharpness of the separation is determined by the reflux ratio. Since the reflux rate is limited by hydraulics considerations (Chapters 14 and 15), higher reflux ratios would require lowering the distillate rate, thus lengthening the distillation cycle. The separation sharpness must therefore be weighed against the time required to complete the distillation. 

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The refluxing solvent provides a constant wash to the reactor and brings back reactants that had escaped from the reaction mixture. The reaction temperature is better controlled by the constant refluxing, and the viscosity of the reaction mixture is lower, which improves the effectiveness of the agitation. The product usually has better color and is more uniform than material made by the fusion process. Ordinarily, the reactor requires no more than... 

FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

Batch Rectification at Constant Reflux Using an analysis similar to the simple batch still, Smoker and Rose [Trans. Am. Inst. Chem. Eng., 36, 285 (1940)] developed the following equation ... 

As an example, consider distilhug at constant composition the same mixture that was used to illustrate constant reflux. The following table is compiled ... 

For preliminary studies of batch rectification of multicomponent mixtures, shortcut methods that assume constant molal overflow and negligible vapor and liquid holdup are useful. The method of Diwekar and Madhaven [Ind. Eng. Chem. Res., 30, 713 (1991)] can be used for constant reflux or constant overhead rate. The method of Sundaram and Evans [Ind. Eng. Chem. Res., 32, 511 (1993)] applies only to the case of constant remix, but is easy to apply. Both methods employ the Fenske-Uuderwood-GiUilaud (FUG) shortcut procedure at successive time steps. Thus, batch rectification is treated as a sequence of continuous, steady-state rectifications. 

The ketone, preferably prepared by a Grignard reaction, was added in such a way as to maintain the ether under constant reflux. When all of the solution had been added, the mixture was refluxed for a further hour. The mixture was then allowed to stand for 12 hours at ambient temperature, after which the reaction mass was extracted with ice and ammonium chloride in known manner. 

Figure 8-32. Batch operations constant reflux ratio and variable overhead composition for fixed number of theoreticai stages/trays. Used and modified by permission, TreybaJi, R. E., Chem. Eng. Oct. 5 (1970), p. 95.

Fixed Number Theoretical Trays Constant Reflux Ratio and Variable Overhead Comporations... 

Batch with Constant Reflux Ratio, Fixed Number Theoretical Plates in Column, Overhead Comporation Varies... 

For a constant reflux ratio, the value can be almost any ratio however, this ratio affects the number of theoretical plates and, consequently, actual trays installed in the rectification section to achieve the desired separation. Control of batch distillation is examined in Reference 134. 

Figure 8-34. Minimum reflux for abnormal equilibrium curve for Batch Operation, constant reflux ratio.

Figure 8-35. Batch distiilation constant reflux ratio after McCabe-Thieie diagram. Revised/adapted and used by pennission, Schweitzer, PA Handbook of Separation Techniques for Chemical Engineers, McGraw-Hiil Book Co. (1979) aiso reprinted by special permission, Chem. Eng. Jan. 23 (1961), p. 134., 1961 by McGraw-Hili, Inc., New York.

Example 8-14 Batoh Distillation, Constant Reflux Following the Procedure of Block [133]... 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Distillation at constant reflux ratio but varying top product composition. 

Figure 14.8 Integration of the Rayleigh Equation for constant reflux ratio. 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

An alternative method of operation is to work with a constant reflux ratio and allow the composition of the top product to fall. For example, if a product of composition 0.9 with respect to the more volatile component is required, the composition initially obtained may be 0.95, and distillation is allowed to continue until the composition has fallen to some value below 0.9, say 0.82. The total product obtained will then have the required composition, provided the amounts of a given purity are correctly chosen. 

These equations enable the final reflux ratio to be determined for any desired end concentration in the still, and they also give the total quantity of distillate obtained. What is important, in comparing the operation at constant reflux ratio with that at constant product composition, is the difference in the total amount of steam used in the distillation, for a given quantity of product, Db. 

If the same column is operated at a constant reflux ratio R, the concentration of the more volatile component in the top product will continuously fall. Over a small interval of time At, the top-product composition with respect to the more volatile component will change from xd to Xd + Axd, where Axd is negative for the more volatile component. If in this time the amount of product obtained is ADb, then a material balance on the more volatile component gives ... 

If the same batch as in Example 11.12 is distilled with a constant reflux ratio of R = 2.1, what will be the heat required and the average composition of the distillate if the distillation is stopped when the composition in the still has fallen to 0.105 mole fraction of ethanol ... 

Figure 11.37. Batch distillation-constant reflux ratio (Example 11.13)...

As the name implies, ratio control involves keeping constant the ratio of two or more flow rates. The flow rate of the "wild or uncontrolled stream is measured and the flow rate of the manipulated stream is changed to keep the two streams at a constant ratio with each other. Common examples include (1) holding a constant reflux ratio on a distillation column, (2) keeping stoichiometric amounts... 

B. 3-Methylheptanoic acid. In a 2-1. three-necked flask fitted with a mercury-sealed stirrer, a reflux condenser carrying a calcium chloride tube, and a dropping funnel are placed 25.0 g. (1.04 g. atoms) of magnesium turnings. The flask is heated to about 100° for a few minutes and then cooled to room temperature. A solution of 178 g. (1.30 moles) of -butyl bromide in 300 ml. of dry ether is prepared and of this solution about 10 ml., together with 30 ml. of dry ether, is run into the flask. The reaction is started by heating to reflux for a few seconds, the stirrer is started, and the remainder of the bromide solution is added at such a rate as to maintain constant reflux (about 1 hour). 

Figure 4.3 Optimizing feed preheat duty at a constant reflux rate.

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